Luminosity Profiles of Prominent Stellar Halos

Home , Disc galaxy, Kuipers, NGC 1055, NGC 4343, NGC 4474, NGC 7814

Journal of the Korean Astronomical Society http://dx.doi.org/10.5303/JKAS.2018.51.4.73 51: 73 ∼ 88, 2018 June pISSN: 1225-4614 · eISSN: 2288-890X c 2018. The Korean Astronomical Society. All rights reserved. http://jkas.kas.org

LUMINOSITY PROFILES OF PROMINENT STELLAR HALOS Hong Bae Ann and Hyeong Wook Park Pusan National University, 2 Busandaehak-ro, Geumjeong-gu, Busan 46241, Korea; [email protected] Received May 9, 2018; accepted June 30, 2018 Abstract: We present a sample of 54 disk galaxies which have well developed extraplanar structures. We selected them using visual inspections from the color images of the Sloan Digital Sky Survey. Since the sizes of the extraplanar structures are comparable to the disks, they are considered as prominent stellar halos rather than large bulges. A single Sérsic profile fitted to the surface brightness along the minor-axis of the disk shows a luminosity excess in the central regions for the majority of sample galaxies. This central excess is considered to be caused by the central bulge component. The mean Sérsic index of the single component model is 1.1 ± 0.9. A double Sérsic profile model that employs n = 1 for the inner region, and varying n for the outer region, provides a better fit than the single Sérsic profile model. For a small fraction of galaxies, a Sérsic profile fitted with n = 4 for the inner region gives similar results. There is a weak tendency of increasing n with increasing luminosity and central velocity dispersion, but there is no dependence on the local background density. Key words: galaxies: general — galaxies: photometry — galaxies: structure

1. INTRODUCTION difficult to observe. In the case of the Milky way, its halo luminosity is ∼ 1 per cent of the total luminosity, Stellar halos are thought to be ubiquitous in disk galax- and other spirals usually have halo luminosities similar ies (Sackett et al. 1994; Morrison et al. 1997; Lequeux to the Milky Way (Courteau et al. 2011). Since the et al. 1998; Abe et al. 1999; Zibetti et al. 2004; Zibetti surface brightness level needed to analyze the structure & Ferguson 2004; Zibetti et al. 2004; McConnachie et −2 of stellar halos is so faint (∼ 30 mag arcsec ), it real. 2006; Chapman et al. 2006; Kalirai et al. 2006; Seth quires highly precise observations and data reductions et al. 2007; de Jong et al. 2007; Mouhcine et al. 2007; are required to prevent observational artifacts. The first Helmi 2008; Jablonka et al. 2010; Ibata et al. 2014). detection of the stellar halo around disk galaxies was In the framework of the Λ cold dark matter (ΛCDM) reported for NGC5907 by Sackett et al. (1994), and cosmology (Springel et al. 2006), the existence of stel- confirmed by Lequeux et al. (1996) and Lequeux et al. lar halos around disk galaxies is very natural because (1998). Zibetti et al. (2004) applied an image stacking they are byproducts of hierarchical galaxy formation. technique to disk galaxies to derive the structural pa- They are thought to be formed by the accretion and rameters of their stellar halos. They showed that stellar disruption of dwarf satellites (Bullock & Johnston 2005; halos are moderately flattened spheroids with spatial Abadi et al. 2006; Font et al. 2006; de Lucia & Helmi luminosity distributions that are well described by a 2008; Font et al. 2008; Johnston et al. 2008; Gilbert et −3 power law (∼ r ). al. 2009; Cooper et al. 2010; Zolotov et al. 2010). There is mounting evidence that the accretion and disruption On the other hand, there are some disk galaxies of infalling satellites play a major role in building the with prominent stellar halos which are bright enough to stellar halos around disk galaxies (Searl & Zinn 1978; be visually identified. A well known example is NGC Johnston et al. 1996; Helmi & White 1999; Ferguson et 4594 (M104), a spiral galaxy, known as the Sombrero al. 2002; Bullock & Johnston 2005; Ibata et al. 2007; galaxy, with a Hubbe type of Sa (de Vaucouleurs et al. Johnston et al. 2008; Romanowsky et al. 2016). On 1991) however, it is sometimes considered as a peculiar the other hand, gas rich mergers in the early phase of elliptical galaxy (McQuinn et al. 2016) because of its the galaxy assembly (Brook et al. 2004), and in − situ bright stellar halo. The nucleus of NGC 4594 is fairly star formation (Font et al. 2011) were also proposed as bright, and there is a supermassive black hole in the cen- explanations of the origin of stellar halos. The ejec- ter (Kormendy 1988, 1996; Ho et al. 1997). More often, tion of in − situ stars formed at high redshift (z ∼> 3) the prominent stellar halo of NGC 4594 is considered as by subsequent major (Zolotov et al. 2009) and minor a massive bulge which contains ∼ 90% of the total lumi- mergers (Purcell et al. 2010) also could contribute to nosity (Kent 1988). However, the V −band minor axis stellar halos. profile of NGC 4594 (Jarvis & Freeman 1985) shows a ∼ The surface brightness of the stellar halos of spi- break at 55 arcsec which suggests two components, a ral galaxies is typically 10 magnitudes below the night stellar halo and central bulge. Detailed surface photom- sky background (Bakos & Trujillo 2012), making them etry of NGC 4594 (Burkhead 1986) also showed that the outer spheroidal component is a prominent stellar halo Corresponding author: H. B. Ann that can be well approximated by an ellipsoid with an 73 74 Ann & Park

of prominent stellar halos and establish basic statistics. In particular, we aim to derive the Sérsic index of the luminosity profile of the prominent stellar halos to char- acterize the shape of the luminosity distribution. The Sérsic index n is known to be correlated with the luminosity, effective radius, and the central velocity dispersion of galaxies (Graham et al. 2001; Mollenhoff & Heidt 2001; Graham 2002) when applied to ellipticals and the bulges of disk galaxies, Since prominent stellar halos are rare phenomena, we are also interested in their environment. This paper is organized as follows. The data and sample selection are described in Section 2, and the results of the present study are given in Section 3. Dis- cussion and conclusions are provided in Section 4.

2. DATA AND SAMPLE SELECTION 2.1. Selection of Prominent Stellar Halos We selected prominent stellar halos using the color images provided by the Sloan Digital Sky Survey (SDSS) Data Release 7 (Abazajian et al. 2009). We used the Korea Institute of Advanced Study Value-Added Figure 1. Color-magnitude diagram of the sample galaxies Galaxy Catalog (KIAS-VAGC; Choi et al. 2010) to se- that show prominent stellar halos. lect the parent sample of target galaxies that have redshifts less than z = 0.032, and r-magnitudes brighter axis ratio of b/a = 0.64. A clear distinction between than 17.77. The number of parent sample galaxies is the central bulge and the stellar halo can be seen in 44,244. We first selected 7,810 edge-on galaxies through the Spitzer 3.6-µ image (Gadotti & Sánchez-Janssem visual inspections of the SDSS color images in the par- 2012), in which the stellar halo is rounder than the cen- ent sample. Because of the large axis ratios (b/a) of the tral bulge, and dominates over the disk and bulge at prominent stellar halos, they are likely to be omitted large radius (r ∼> 215 arcsec). in the sample of edge-on galaxies selected by axis ratio Since most stellar halos are too faint to be observed criteria. Thus, it is necessary to conduct a visual inspec- by conventional methods, the number of galaxies with tion of the color images to see whether or not galaxies detailed observations of their stellar halos is quite lim- are edge-on. The selection criterion we have applied is ited. Therefore, there are no statistical samples of the the axis ratio of the disks and not the entire galaxy. structure and physical properties of stellar halos. On Sometimes, a well distinguished dustlane is considered the other hand, stellar halos have been well predicted as evidence of edge-on disks. by numerical simulations (Katz & Gunn 1991; Stein- Since we have selected the prominent stellar halos metz & Muller 1995; Brook et al. 2004) in the CDM by visual inspections of the color images without quan- scenario (White & Frenk 1991; Kauffmann et al. 1993). titative selection criteria, the selected sample is subject In a sense, these simulations failed to reproduce Milky to some personal bias, but we do not think we omit a Way type stellar halos whose mass fraction is less than significant fraction of prominent stellar halos. The final ∼ 1 per cent of the total luminous matter. Most stel- sample contains 54 galaxies, and corresponds to 0.1% lar halos that they modeled were too massive compared of the parent sample of 44,244 galaxies, and 0.7% of to observed stellar halos. However, Brook et al. (2004) the 7810 edge-on galaxies. This means that prominent was able to make different kinds of stellar halos depend- stellar halos are very rare. The basic properties of the ing on the supernova (SN) feedback mechanisms used. 54 galaxies are presented in Table 1, most of which are One model is similar to the faint stellar halos, often obtained from the KIAS-VAGC. observed, and another is similar to the massive stellar Figure 1 shows the color-magnitude diagram of halos that were reported in the previous studies (Katz galaxies in the final sample. Mr is derived from the & Gunn 1991; Steinmetz & Muller 1995). Although r−band model magnitude corrected for Galactic ex- prominent stellar halos can easily be observed by con- tinction using the r−band extinction values from SDSS ventional methods, there is no extensive study of the DR7. A K-correction and an evolution correction are prominent stellar halos due to their rarity in the local not applied. Distances to the galaxies were derived from universe. Observations of prominent stellar halos, how- the redshifts, corrected for the motion relative to the ever, seem to be a promising path to understanding the centroid of the local Group (Mould et al. 2000). For structural properties of stellar halos. This study will galaxies with z < 0.01, the metric distances provided uncover a wealth of information about the formation by NED are used. The galaxies lying inside a 10◦-cone and evolution of galaxies. around M87 with redshift less than z = 0.007 are as- The purpose of this study is to construct a sample sumed to be the members of the Virgo Cluster, and the Luminosity Profiles of Prominent Stellar Halos 75

Figure 2. Images of disk galaxies that show prominent stellar halos. The image sizes are adjusted to ﬁt to the frame size. 76 Ann & Park

Figure 2. Continued. Luminosity Profiles of Prominent Stellar Halos 77

Figure 2. Continued.

distance of the Virgo cluster is used for those galaxies. 3. RESULTS We assumed the Virgo distance as D = 16.7 Mpc and −1 −1 3.1. Morphological Properties of Prominent Stellar H=75 km s Mpc . It is evident that the galaxies Halos with prominent stellar halos have photometric properties similar to the galaxies in the red sequence defined Figure 2 shows the color images of the 54 sample galax- by early type galaxies, particularly by elliptical galaxies from the SDSS DR7. At first glance, the promi- ies. nent stellar halos are mostly flattened ellipsoids. They have somewhat red colors, typical of old stellar popula- 78 Ann & Park tions, and sizes comparable to their disks. Some halos have sizes about half that of their disk, while some halos are nearly spherical. For the majority of sample galaxies dust lanes can be observed, some of which are offset from the central bulge, and middle plane of the disk. One of the remarkable morphological properties of galaxies with prominent stellar halos is that they are early type disk galaxies, mostly S0 and S0/a galaxies, without well developed bulges. Most of the luminosity around the disk come from the stellar halo, and not from the bulge. The bulge luminosity is confined to the central regions. Of course, as shown in previous studies (Hes & Peletier 1993), the prominent structures around the disk plane can be considered as an extended bulge. However, as will be shown below, the extraplanar prominent structures are considered here as stellar halos. As can be seen in Figure 2, most galaxies with prominent stellar halos have red disks except for the two extremely blue disks (NGC 3413 and NGC 5672) and show well distinguished dustlanes. There are several galaxies whose disk morphology differs from that Figure 3. Frequency distribution of axis ratios of the disk of others. They have disks with somewhat blue col- galaxies which have prominent stellar halos, compared to ors in the outer parts, indicating spiral arms (2MASX elliptical galaxies in the local universe (z ∼< 0.01). Solid J00224457+1456586, UGC04332, CGCG 091-099, NGC lines represent the galaxies with prominent stellar halos and 4343, CGCG 191-031, UGC 10205). Warped disks are dotted lines indicate the local elliptical galaxies. observed in a small fraction of sample galaxies. Figure 3 shows the histogram of the axial ratios los, the luminosity of the stellar halo is larger than that (b/a) of the sample galaxies along with the axial ratios of the bulge. In particular, the luminosity distribution of a sample of elliptical galaxies in the local universe at radii larger than a few times the effective radius of (z ∼< 0.01) from the catalog of Ann et al. (2015). The the bulge is dominated by the stellar halo. The surface axis ratios of the sample galaxies were taken from the brightness profiles along the minor-axis of the sample KIAS-VAGC (Choi et al. 2010) where the major- and galaxies are examined to see whether the sample galax- minor-axis length were derived from the i-band isopho- ies have two components. Breaks in the surface bright- tal maps. the two types of galaxies in the local universe ness profiles are considered as the signature of multiple show a similar distribution of axis ratios. Since the components. The majority of the sample galaxies are lengths of the major axis of the stellar halos and the found to have a break at r ∼< 10 arcsec from the center, disks are similar, the axial ratios of the sample galax- indicating the existence of a central bulge. ies are determined by the shape of the halos. Thus, Several functional forms are commonly used to the similarity between the two axis ratio distributions represent the luminosity profiles of galaxies. In the suggests that the prominent stellar halos are flattened early era of galaxy studies, the Hubble profile, I(r) = ellipsoids, similar to the elliptical galaxies. 2 I0/(1 + (r/rc) ), was employed to represent the lumi- 3.2. Luminosity Profiles of the Prominent Stellar Halos nosity distribution of elliptical galaxies. Later, de Vau- couleurs (1958) introduced the r1/4-law to represent the Surface photometry of the r-band images for the sam- luminosity distributions of the bulge of spiral galaxies ple galaxies is used to derive the luminosity profiles of as well as the elliptical galaxies. Freeman (1970) intro- the prominent stellar halos. We used the mode of pixel duced exponential function to describe the luminosity values around the target galaxy as the sky background, distribution of the disks of spiral galaxies. These func- Isky , to derive the galaxy intensity distribution given by tions have been used very successfully to describe the − Ig = Iobs(x, y) Isky . We used the SPIRAL (Ichikawa et luminosity distributions of ellipticals and spirals. Sérsic al. 1987; Okamura 1988) to extract the surface bright- (1968) introduced a generalized function to represent ness profiles along the minor-axis, where the luminosity luminosity distribution of galaxies including both ellip- of the stellar halo is less affected by the disk luminosity. ticals and spirals. The Sérsic function is defined by The effect of dustlanes is also negligible in the minor- three parameters, Ieff , reff , and n as follows axis profiles, especially in the halo regions. 1/n The bulge and halo are the main contributors to I(r)= Ieff exp(−bn((r/reff ) − 1) the luminosity profile along the minor-axis. In most normal disk galaxies the contribution of halo component where reff and Ieff are the effective radius and effec- is negligibly small compared to that of the bulge. But, tive intensity (which is the intensity at r = reff ), re- in the case of disk galaxies with prominent stellar ha- spectively. The parameter n is the Sérsic index. The Luminosity Profiles of Prominent Stellar Halos 79 constant bn is approximated by bn = 1.9992n − 0.3271 in the bulge fitting range affects the decomposition sig- (Caon et al. 1993; Prugniel & Simen 1997). The de nificantly. Thus, the bulge fitting range is selected in- Vaucouleurs law and exponential law are the cases for teractively by minimizing the residuals of the fit. At n = 4 and n = 1, respectively. first, n = 4, (i.e., de Vaucouleurs law), was assumed for As there is no commonly used functional form for the bulge, and n was varied for the halo. Using these the luminosity profile of prominent stellar halos, both parameters the fittings were unsuccessful for the major- the Hubble profile and the Sérsic profile are examined ity of the sample galaxies. Various alternative values of to determine which is more representative of the lumi- n were explored for the bulge, and the Sérsic index of nosity distribution. The Hubble profile is known to rep- n ≈ 1 is found to well represent most galaxies. Since resent the luminosity distribution of the stellar halos of n = 1 gives successful fits for most of the sample galax- edge-on galaxies obtained by image stacking techniques ies, n = 1 is used as the default Sérsic index for the (Zibetti et al. 2004), However, it was found that the bulge component. However, n=4 was adopted if n =4 Hubble profile does not fit well to the observed luminos- gives a similar result. In general, the Sérsic index of ity distributions of the majority of the sample galaxies the stellar halo in the two component model becomes in our sample. On the contrary, the Sérsic profile better smaller than that of the single component model. The represents the luminosity distribution of the prominent Sérsic index of the two component models nb (bulge) stellar halos. and nh (halo) are listed in Table 2. Figure 4 shows the results of fitting a single Sérsic It is interesting to note that about 15% of the sam- profile along the minor-axis. A χ2 minimization tech- ple galaxies have shell structures outside of the visi- nique is applied to derive the three free parameters, n, ble stellar halo. The presence of the shell structure reff and Ieff , with fitting ranges selected from the vi- is indicated by the large bump in the minor-axis pro- sual inspection of the minor-axis profiles. The bulge files. The galaxies with well developed outer shell struc- dominated regions are avoided during fitting a single ture are NGC 0442, CGCG 387-052NED0, NGC 3999, Sérsic profile to the observed profiles. The resulting NGC 4082, CGCG 098-103, CGCG 244-046, NGC 5463, profiles are insensitive to the fitting ranges except in 2MASX J14443669+1631314, CGCG 076-136. the far outer regions where the surface brightness is affected by the background sky or by other components 3.3. Distribution of the Sersic´ Index such as rings. For some galaxies, there are wiggles or Figure 5 shows the frequency distribution of the Sérsic bumps at large radii which are due to the contamina- index for the 54 stellar halos derived by fitting a single tion from nearby objects or in the case of large bumps Sérsic function to the surface brightness profiles along due to outer rings or shell structures. the minor-axis of the sample galaxies. Since the detailed At first glance, the observed luminosity distribu- shape of the frequency distribution depends on the bin tions of prominent stellar halos are well represented by size, due to the small sample size, the frequency distri- the Sérsic profiles. As shown in Figure 5, the fitted butions using different bin sizes were examined. It was Sérsic index n varies from 0.5 to 3.5 with a peak near found that the bin size of ∆n =0.3 is optimum choice n =1.2. About 90% of the sample galaxies have n less for the present sample. It keeps the general shape of the than 3. For the majority of galaxies. there is a luminos- frequency distribution, while suppressing the statistical ity excess over the halo profile at small radii. This cen- noise. tral luminosity excess is mostly due to the bulge which It appears that there are at least two peaks, one is not taken into account in the profile fittings. There peak at n ≈ 1.2, and the other at n ≈ 3.4. The number are 46 galaxies in the sample(85%) that show a clear of galaxies around the first peak at n ≈ 1.2 is 37 and luminosity excess. Since the luminosity excess is due it comprises ∼ 70% of the total sample. The number to the luminosity of the bulge, the luminous structures of galaxies near the second peak at n ≈ 3.4 is 4, which around the disk plane must be stellar halos. Thus, this is less than 10% of the total sample. The remaining study presents a statistically significant sample of the galaxies, comprising the remaining quarter of the total structures of stellar halos for the first time. The stel- sample, are located near a third peak at n ≈ 2. Since lar halos are prominent enough to be visually identified it is not clear whether the third peak at n ≈ 2 is a from conventional images, such as SDSS. The three pa- real feature, the sample galaxies are divided into two rameters of the single component Sérsic profile are listed groups. One group is comprised of the galaxies around in Table 2. the first peak at n ≈ 1.2, including the galaxies near Since the majority of sample galaxies show a cen- the third peak at n ≈ 2, and the other group consists tral luminosity excess, which is assumed to be due to of galaxies around the second peak at n ≈ 3.4. bulge component, the luminosity distributions along the The number of galaxies in the larger group is 50 minor-axis of sample galaxies are also fitted by two which comprises 93% of the total sample. The mean Sérsic functions, one for the bulge and the other for Sérsic index of this group is 1.42 ± 0.55. If we exclude the halo. The iterative fitting technique (Kormendy the galaxies near the third peak at n ≈ 2, it becomes 1977) is applied to decompose the bulge and halo si- 1.19 ± 0.35, thus very close to the exponential profile. multaneously. The iterative profile decomposition tech- The distribution of the Sérsic index of this group is nique assumes fitting ranges for each component where similar to that of the dwarf elliptical/spheroidal galaxies one component dominates the other. A small change (Binggeli & Jerjen 1998; Ryden et al. 1999; Grant et al. 80 Ann & Park

Table 1 Basic properties of 54 disk galaxies with prominent stellar halos.

a b c d ID α2000 δ2000 D Mr u − r g − r mc b/a σ Galaxy name 354822 0.812042 16.145420 12.6 -19.92 3.37 1.28 SA(s)ab? 0.25 0.0 NGC 7814 381062 5.685914 14.949751 77.0 -19.89 2.19 0.76 Sa 0.81 98.1 J00224457+1456586 2092266 18.661020 -1.020657 56.4 -21.34 2.97 1.04 S0/a? 0.64 176.4 NGC 0442 323194 27.295120 -10.426420 17.8 -19.98 2.96 1.06 SAB(s)ab 0.68 108.0 NGC 0681 1742502 32.784565 -0.654782 60.3 -20.76 2.51 0.85 Sb 0.70 125.1 CGCG 387-052NED02 809401 37.040436 0.799052 74.7 -21.01 2.83 0.97 S0/a 0.90 193.5 CGCG 388-020 1767358 40.438672 0.442579 10.2 -18.31 1.64 0.88 SBb? 0.52 74.5 NGC 1055 764571 47.772167 -0.546510 84.0 -20.51 2.34 0.78 S0/a 0.74 127.7 CGCG 390-014 326159 61.230087 -6.304820 80.1 -20.96 2.87 0.99 1 0.89 0.0 J04045523-0618170 1728488 116.389717 45.772331 94.8 -21.48 2.92 1.01 S0 0.83 180.6 CGCG 235-036 1905088 121.556000 17.706573 45.8 -21.33 3.06 1.06 S0/a 0.53 273.9 NGC 2522 1875243 124.907845 21.114300 53.7 -20.65 3.38 0.98 S? 0.72 176.1 UGC 04332 225814 132.721054 55.419109 91.6 -20.74 2.76 0.94 S0 0.73 184.5 J08505302+5525081 2257869 142.595779 19.469234 42.4 -19.74 2.31 0.92 2 0.93 71.6 CGCG 091-099 78806 148.334244 0.697720 35.7 -20.93 2.94 1.03 S0 0.70 180.4 NGC 3042 1248278 153.612778 40.945045 68.5 -19.98 3.01 1.11 S0/a 0.68 125.8 J10142710+4056420 1960169 162.291870 29.815895 94.0 -21.35 2.93 1.03 1 0.63 0.0 CGCG 155-021NED02 2204882 162.817932 27.975279 13.9 -19.84 2.85 0.97 S0 0.64 0.0 NGC 3414 1894107 162.836288 32.766331 6.1 -16.07 2.27 0.38 S0 0.57 0.0 NGC 3413 2221022 173.614166 25.876469 93.3 -21.39 2.92 1.04 1 0.63 186.3 IC 0710 965668 173.901520 54.948639 59.0 -21.50 2.76 0.94 SB0 0.77 202.4 NGC 3737 2356878 174.083298 20.521761 63.0 -20.33 2.91 1.02 1 0.72 228.3 CGCG 126-108 873264 175.686890 52.779636 57.6 -20.95 2.84 1.01 SA(s)a? 0.43 146.0 NGC 3824 253847 176.162704 67.955879 28.8 -18.91 2.61 0.92 S? 0.70 74.6 UGC 06714 2205307 177.191284 29.641171 68.4 -20.44 2.81 1.02 2 0.59 0.0 MCG +05-28-032 901271 177.644882 50.529064 69.8 -21.29 2.77 0.93 S0/a 0.58 203.3 UGC 06811 2322850 179.485378 25.068284 41.0 -18.58 2.62 0.88 1 0.69 87.1 NGC 3999 2333089 180.823410 20.636995 61.5 -20.10 2.74 0.95 1 0.60 117.8 J12031760+2038134 1061991 181.297729 10.670613 68.5 -20.66 2.91 1.05 Sbc 0.83 146.0 NGC 4082 2247528 182.475800 25.311249 68.5 -20.50 2.76 0.95 1 0.80 166.9 CGCG 128-04 2414948 183.345688 16.957827 83.9 -20.58 2.81 0.93 1 0.55 185.4 CGCG 098-103 2311827 183.505875 23.010292 72.7 -19.75 2.78 0.96 2 0.77 114.5 J12140140+2300370 1202224 184.334381 46.635284 71.4 -20.02 2.72 0.90 S0/a 0.90 187.7 J12172024+4638068 2319495 184.743393 24.186310 72.8 -20.75 2.93 1.03 2 0.62 200.2 IC 3141 1288874 185.914588 6.951942 9.1 -18.92 3.00 1.02 SA(rs)b? 0.63 101.0 NGC 4343 1828259 187.473083 14.068640 15.2 -19.47 2.63 0.91 S0 0.56 0.0 NGC 4474 959348 190.697632 11.442470 10.9 -19.81 2.80 0.95 S0 0.53 0.0 NGC 4638 1176719 193.205093 47.685013 96.5 -20.92 3.04 1.05 1 0.55 0.0 CGCG 244-046 1227719 200.125336 43.083969 13.7 -18.55 2.71 0.89 S? 0.86 0.0 NGC 5103 1989264 207.772507 25.093710 87.5 -21.97 2.79 0.93 1 0.79 0.0 UGC 08763NED02 49109 208.428314 0.060874 88.4 -21.52 2.89 0.99 S0/a 0.86 221.2 CGCG 017-093 1838833 210.311371 37.882977 74.2 -20.64 2.72 0.95 Sa 0.55 128.5 CGCG 191-031 1340227 211.543945 9.353449 72.2 -21.98 2.76 0.94 S? 0.55 0.0 NGC 5463 2380372 213.913589 14.282601 55.3 -21.51 2.89 0.98 S0 0.67 226.5 NGC 5525 1348148 217.970367 7.320885 81.3 -21.03 3.19 1.12 2 0.69 173.3 CGCG 047-080 1913169 218.159805 31.670155 36.2 -19.69 1.71 0.56 Sb? 0.95 0.0 NGC 5672 980767 220.880341 49.393093 91.1 -21.05 2.25 0.82 Sa 0.83 137.4 SBS 1441+496 1430278 225.568130 11.917582 97.1 -21.81 2.98 1.03 Sa 0.85 203.8 CGCG 076-136 2412381 229.180084 55.409248 34.9 -20.75 3.14 1.11 SA(s)b 0.56 111.9 NGC 5908 1423817 241.207611 8.481091 52.8 -19.94 2.65 0.91 1 0.64 120.3 CGCG 079-039 1393362 241.667435 30.099058 67.1 -21.66 2.59 0.95 Sa 0.58 165.2 UGC 10205 2001255 243.569672 17.757429 91.1 -21.72 3.07 1.07 Sa 0.65 212.5 NGC 6084 532728 249.074478 44.135700 96.0 -21.70 2.95 1.01 S0/a 0.86 220.8 CGC G224-076 726224 328.157928 12.535856 89.1 -21.55 2.93 1.06 E 0.67 222.9 CGCG 427-032 a Galaxy distance in unit of Mpc. b Morphological types form the NASA Extragalactic Database. If not available, the KIAS-VAGC morphological class is given: 1 (E/S0) and 2 (Sp/Irr). c Central velocity dispersion in unit of km s−1. In case of no data, we use 0.0. d Galaxy name taken from the NASA Extragalactic Database. We omit ‘2MASX’ for the 2 Micron All Sky Survey Extended objects names. Luminosity Profiles of Prominent Stellar Halos 81

0 10 20 30 0 10 20 30 0 10 20 30 0 10 20 30 R (arcsec) R (arcsec) R (arcsec) R (arcsec) Figure 4. Single component Sérsic function fit to the minor-axis profiles of galaxies that show prominent stellar halos. The circles indicate the data points while the solid lines are the fitting result.

2005; Kim et al. 2006; Janz et al. 2014; Seo & Ann cess show double peaks, with one peak at n ≈ 1.3 and 2018). the other peak at n ≈ 2.8. There are only two galaxies (4.3%) that have n larger than 2.4 among galaxies Figure 6 shows the frequency distribution of the with a central luminosity excess, while 63% of galaxies S´ersic index of prominent stellar halos with (solid lines) without central luminosity excess have n larger than and without (dotted lines) the central luminosity ex- 2.2. Thus, the extraplanar prominent structure with- cess, respectively. The prominent stellar halos of the out central luminosity excess can be thought of as the sample galaxies with a central luminosity excess have extended bulges. small n, while those without a central luminosity ex- 82 Ann & Park

10 20 30 0 10 20 30 0 R (arcsec) R (arcsec) 5

0 10 20 30 0 10 20 30 R (arcsec) R (arcsec) Figure 4. Continued.

3.4. Dependence on the Physical Properties of Galaxies ure 7 shows the distribution of n as a function of Mr (lower panel) and σ (upper panel). As shown in the The dependence of the Sérsic index n, derived from the lower panel of Figure 7, there is no clear correlation be- single-component fitting, on the two observable phys- tween the Sérsic index and the luminosity. There may ical parameters, the r-band absolute magnitude (Mr), be, however, a tendency of smaller n for less luminous and the velocity dispersion (σ), are investigated. Both galaxies (Mr > −20). More specifically, there are no parameters are closely related to the mass of a galaxy, prominent stellar halos that have n larger than ∼ 2 for which is a key parameter that controls the formation galaxies with Mr > −20, but prominent stellar halos and evolution of a galaxy (Peng et al. 2012). Fig- with n larger than ∼ 2 are observed in bright galaxies Luminosity Profiles of Prominent Stellar Halos 83

Figure 5. Frequency distribution of the Sérsic index for the Figure 6. Frequency distribution of the Sérsic index for the single component model. single component model grouped by the presence/absence of a central luminosity excess. Galaxies with a central excess are represented by the solid line while those without central excess are plotted by dotted line. (Mr < −20). However, even for bright galaxies, their ≈ stellar halos are likely to have the Sérsic index of n 2. 3.5. Local Background Density The upper panel of Figure 7 shows the distribution The local background density (Σ) is used to examine the of n as a function of σ. There is no strong correlation environmental dependence of the Sérsic index. The lo- between the two parameters, but there is a tendency cal background density can be derived using a variety of of increasing n with increasing σ. It is also apparent approaches (Muldrew et al. 2012), but the nth nearest that there are no prominent stellar halos that have a neighbor method with n = 5 is used here. The defini- large Sérsic index (n > 2.2) for galaxies with velocity tion of neighbor galaxy by Ann (2017) is used where a dispersions less than σ = 150 km s−1. Thus, the de- neighboring galaxy is defined to have a velocity differ- pendence of the Sérsic index on the velocity dispersion ence less than the linking velocity ∆V = 500 km s−1, is very similar to the dependence of the Sérsic index on and a luminosity brighter than the limiting magnitude the luminosity. of the volume-limited sample. The background density using the projected distance to the 5th nearest galaxy The structure of a galaxy formed through major is defined as , mergers is different from that formed through a mono- 5 lithic collapse of galactic proto-clouds. Giant ellipsoidal Σ5 = 2 , 4πr 5 structures (Sérsic index of n ≈ 4) are likely to be made p, by violent mergers, while disks (Sérsic index of n ≈ 1) where rp,5 is the projected distance to the 5th near- are made by dissipational collapse of proto-clouds that est neighbor galaxy. The local background density is have non-negligible angular momentum. The absence of normalized by the mean background density (Σ¯5). large Sérsic index (n> 2.2) for galaxies with Mr > −20 Figure 8 shows the histogram of the local back- or σ< 140 km −1, i.e, relatively low mass galaxies, sug- ground density of the prominent stellar halos along with gests that major mergers do not play a significant role that of SDSS galaxies with redshift less than z =0.04. for less luminous galaxies with prominent stellar halos. There is not much difference between the two distribu- It is worth noting that our dividing value of n = 2.2 tions. But, the present sample shows somewhat larger is the same as the critical value suggested by Fisher & fractions at high density regions and smaller fractions Drory (2008) who used this value to divide pseudob- at low density regions than the full sample. If we divide ulges from classical bulges. If galaxies with small Sérsic the full sample into the blue galaxies and red galaxies, indices did not form through major mergers, then the the local background density of the present sample is majority of prominent stellar halos must be caused by more similar to that of red galaxies. The low density other mechanisms. This is because ∼ 70% of prominent tail of the present sample is due to the spiral galaxies. stellar halos have a Sérsic index less than the critical Since there is a tight relationship between the lu- value, regardless of luminosity or mass. minosity of a galaxy and the local background density 84 Ann & Park

Table 2 S´ersic index of prominent stellar halos.

a b ′ c ID n ueff reff nb nh Σ5 354822 1.3 -0.16 1.84 1.0 0.6 -1.470 381062 1.4 2.56 4.82 4.0 1.7 -1.980 2092266 2.0 -0.07 2.39 1.0 0.8 0.142 323194 0.9 0.54 2.15 1.0 0.5 -0.399 1742502 1.3 0.74 2.35 1.0 0.3 -0.138 809401 0.7 1.48 3.96 1.0 0.4 -0.428 1767358 0.7 0.68 2.32 1.0 0.5 -0.255 764571 0.9 0.46 2.19 0.9 0.9 -0.063 326159 0.5 1.41 3.72 1.0 0.4 -1.670 1728488 0.6 1.82 6.58 1.0 0.4 0.040 1905088 1.5 0.08 1.74 1.0 0.7 0.187 1875243 0.6 1.08 3.88 1.0 0.4 0.718 225814 1.0 1.33 3.36 1.0 0.6 1.290 2257869 1.4 1.07 2.64 4.0 1.2 -0.425 78806 1.3 -0.12 1.74 4.0 1.3 -1.030 1248278 1.5 0.93 2.62 1.0 0.9 -0.347 1960169 0.9 0.40 3.77 1.0 0.5 0.579 2204882 1.2 -0.40 1.48 1.0 0.5 -0.015 1894107 2.5 -0.56 0.22 1.0 0.8 -0.143 2221022 1.2 0.44 3.93 1.0 0.5 0.186 965668 1.8 0.71 4.23 4.0 0.9 1.310 2356878 2.3 1.54 3.68 1.0 0.5 0.627 873264 1.1 0.05 2.60 1.0 0.8 -0.658 253847 2.1 0.14 0.86 1.0 0.3 0.007 2205307 0.9 0.19 2.29 1.0 0.2 -0.362 901271 3.2 -0.86 2.03 1.0 0.2 0.790 2322850 2.1 1.39 1.23 1.0 1.4 1.820 Figure 7. Correlations between physical parameters and the 2333089 0.9 0.55 1.80 1.0 0.5 0.757 Sérsic index for the single component model. n-σ in the 1061991 3.3 2.92 3.92 1.0 0.3 -0.706 upper panel and n-Mr in the lower panel. 2247528 3.5 1.01 2.52 1.0 0.3 0.844 2414948 1.6 1.22 3.50 1.0 1.0 -0.394 (Goto et al. 2003; Park et al. 2007), we used galaxies 2311827 1.5 1.57 3.07 1.0 0.7 -0.167 1202224 1.3 1.10 2.52 1.0 0.5 0.218 brighter than Mr = −20 to examine the environmental dependence of the Sérsic index. Fig. 9 shows the 2319495 2.2 2.20 2.70 0.2 1.0 -0.001 1288874 1.2 -0.43 0.41 0.2 1.0 1.830 frequency distribution of the local background density 1828259 2.0 0.76 1.47 4.0 1.3 1.120 for prominent stellar halos for galaxies brighter than 959348 1.6 2.06 1.49 4.0 10.0 1.060 − Mr = 20. The distribution of prominent stellar halos 1176719 2.8 -1.02 1.73 1.0 1.0 0.338 with n< 2.2 is similar to that of the background galaxy 1227719 0.9 -0.26 0.65 1.0 0.4 -0.409 distribution, while the distribution of the prominent 1989264 1.8 0.25 5.30 1.0 0.6 1.740 stellar halos with n > 2.2 shows a much narrower dis- 49109 1.3 0.65 3.30 1.0 0.6 -0.052 tribution, with maximum frequency at Σ5/Σ¯5 = 0.75. 1838833 0.8 0.65 3.06 1.0 0.4 -0.547 However, due to the small sample size for the galaxies 1340227 2.3 1.01 3.93 0.2 0.4 0.710 with large n, this result could be due to statistical noise. 2380372 1.7 0.45 4.71 1.0 0.6 -0.358 We carried out a K-S test and found that there is no 1348148 3.6 0.14 2.66 1.0 1.0 0.100 1913169 1.8 1.05 2.43 0.2 1.0 -0.064 significant difference between the two groups. The K-S 980767 1.5 1.17 4.17 1.0 0.4 -0.560 test gives p =0.21. 1430278 1.4 0.83 5.30 0.2 1.0 0.137 2412381 1.0 0.24 2.66 1.0 0.4 0.450 4. DISCUSSION AND CONCLUSIONS 1423817 1.1 0.62 1.95 1.0 1.2 -0.524 We have analyzed the luminosity profiles along the 1393362 2.2 -0.51 3.71 1.0 0.4 -0.660 minor-axis of disk galaxies with luminous halos. Many 2001255 2.3 3.94 4.12 0.2 1.0 -0.630 previous studies considered the prominent stellar halo 532728 1.2 0.71 5.41 1.0 0.6 1.120 to be an extended bulge, which is well fitted by the 726224 1.1 0.58 3.91 1.0 0.8 1.100 1/4 a r -law (Wainscoat et al. 1990; Emsellem et al. 1996). Effective surface brightness - µsky. However, in this study it is treated as a stellar halo b Effective radius in unit of arcsec. c ′ ¯ which is luminous enough to be observed. The reason Σ5 is defined as log(Σ5/Σ5). for this is that most of the sample galaxies have bright central regions that are assumed to be the central bulge light in the surface brightness profile along the minor- component. They are represented by a central excess of axis when the observed profile is fitted by a single Sérsic Luminosity Profiles of Prominent Stellar Halos 85

Figure 8. Frequency distribution of the local background Figure 9. Frequency distribution of the local background density of the prominent stellar halos (solid lines) compared density of the prominent stellar halos with n < 2.2 (solid with SDSS galaxies that have redshift less than z = 0.04. lines) compared with the cases for n> 2.2. We used galaxies brighter than Mr = −20. profile. The average Sérsic index n of the single component fits is n = 1.1 ± 0.9, and the majority of sam- of pseudobulges is not expected to dominate over the ple galaxies have n less than ∼ 2.2. The small n also disk luminosity. supports the assumption that the extended regions are We have examined the dependence of the Sérsic in- prominent stellar halos rather than extended bulges. In dex of prominent stellar halos on the physical properties the literature, there are some studies that consider the of galaxies and their environment. The physical prop- luminous spheroidal component as a prominent stellar erties considered here are the galaxy luminosity (Mr) halo (Harris et al. 1984; Burkhead 1986; Wagner et al. and the central velocity dispersion (σ). As shown in − 1989) although some authors have not explicitly used Figure 7, galaxies with low luminosities (Mr ∼> 20) or −1 the terminology spheroid (Burkhead 1986). low velocity dispersions (σ ∼< 150 km s ) have Sérsic The Sérsic index n of prominent stellar halos is indices smaller than n ≈ 2.2, while galaxies with high quite different from that of classical bulges. Approx- luminosities and high velocity dispersions have a wide imately 80% of prominent stellar halos in the sam- range of Sérsic indices including n > 2.2. If a large ple have n smaller than the critical value of n = 2.2, Sérsic index is considered to be related to the sub- which divides the bulges of disk galaxies into classi- structures formed through merger induced violent re- cal bulges and pseudobulges (Fisher & Drory 2008). laxation, then the prominent stellar halos of massive Classical bulges have n greater than 2.2, and pseudob- galaxies must have origins different to those of less mas- ulges have n smaller than 2.2. Given that the Sérsic sive galaxies. One plausible scenario for the origin of the index n is a shape parameter which is closely related prominent stellar halos that have n smaller than 2.2 is to the galaxy formation mechanism, the majority of the intensive in − situ star formation. The fact that the stellar halos are formed by mechanisms different there is no environmental dependency on the prominent from that for the classical bulge formation. Prominent stellar halos supports the above scenario, since super- stellar halos do not seem to have formed from major nova explosions are an internal process which does not mergers followed by violent relaxation because result- depend on the environment. ing substructures would have large n. If the luminous In the ΛCDM cosmology, the stellar halos of disk spheroidal components are bulges, they are likely to galaxies are formed by the debris of accreted satellites be pseudobulges because of their small n, since pseu- (Bullock & Johnston 2005; Read et al. 2006; Sales et al. dobulges are thought to be made through the secular 2007; Font et al. 2008; de Lucia & Helmi 2008; Johnston evolution of disk galaxies, driven by non-axisymmetric et al. 2008; Font et al. 2011) as well as in − situ halo potential such as a bar (Kormendy & Kennicutt 2004; stars (Cooper et al. 2015). Halo stars formed in − situ Okamoto 2013). However, luminous structures around include heated disk stars which are ejected into the halo. the disks are not considered to be pseudobulges because These heated disk stars make a negligible contribution of their extent which is comparable to the disks. Since to the total halo mass (Cooper et al. 2015), but they pseudobulges are made of disk material, the luminosity contribute significant fractions in the inner halo, simi- 86 Ann & Park lar to the fraction of stars from satellite debris (Purcell Caon, N., Capaccioli, M., & D Onofrio, M. 1993, On the et al. 2010; Font et al. 2011; McCarthy et al. 2011). Shape of the Light Profiles of Early Type Galaxies, MN- The disrupted debris of accreted satellites and in−situ RAS 265, 1013 halo stars are a good explanation for the faint stellar Chapman, S. C., Ibata, R., Lewis, G. F., Ferguson, A. M. halos such as that of the Milky Way or M31. How- N., Irwin, M., McConnachie, A., & Tanvir, N. 2006, A ever, they can not account for the prominent stellar Kinematically Selected, Metal-Poor Stellar Halo in the Outskirts of M31, ApJ, 653, 255 halos presented in this study, because the luminosity Choi, Y.-Y., Han, D.-H., & Kim, S. S. 2010, Korea Institute of the prominent stellar halos is brighter than the disk for Advanced Study Value-Added Galaxy Catalog, JKAS, luminosity. Since the present sample of prominent stel- 43,191 lar halos is observed in early-type disk galaxies, mostly Cooper, A. P., Cole, S., Frenk, C. S., et al. 2010, Galactic S0 and S0/a, the cause of prominent stellar halos are Stellar Haloes in the CDM Model, MNRAS, 406, 744 thought to be related to the same feedback mechanism Cooper, A. P., Parry, O. H., Lowing, B., Cole, S., & Frenk, that produces lenticular galaxies. The thermal feedback C. S. 2015, Formation of in − situ Stellar Haloes in Milky model by Brook et al. 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